Aluminum nitride transitional layer for reducing dislocation density and cracking of AlGaN epitaxial films

ABSTRACT

A denticulated Group III nitride structure that is useful for growing Al x Ga 1-x N to greater thicknesses without cracking and with a greatly reduced threading dislocation (TD) density.

This application is a divisional application of the prior-filed U.S. nonprovisional patent application Ser. No. 11/504,885, filed on Aug. 15, 2006, now U.S. Pat. No. 7,915,626, and claims priority benefit therefrom. This prior-filed application is hereby incorporated by reference.

The United States Government has rights in this invention pursuant to Department of Energy Contract No. DE-AC04-94AL85000 with Sandia Corporation.

BACKGROUND OF THE INVENTION

The desire for UV-emitting photonic devices has produced much activity in the growth of Al_(x)Ga_(1-x)N-based devices. A typical growth process by the metal-organic chemical vapor deposition (MOCVD) method for aluminum nitride or aluminum gallium nitride consists of the growth of a thin nucleation layer (typically less than 10 nm) at a temperature typically between 400° C. and 700° C. Common precursors when the nucleation layer is AlN are ammonia and trimethyl aluminum (TMA) mixed together in the deposition chamber or separately pulsed into the deposition chamber; typical pressures are between 40 and 150 Torr. Following the formation of this very thin nucleation layer, a thicker AlN or high-Al-content Al_(x)Ga_(1-x)N layer is grown at temperatures in excess of 1050° C. and at pressures typically below 150 Torr. Growth rates of the high-temperature AlN or Al_(x)Ga_(1-x)N layer under these conditions are typically near or less than about 1 micrometer per hour. For AlN films on sapphire substrates, threading dislocation densities with an edge or mixed edge+screw character are typically very high, greater than 1×10¹⁰/cm². Much lower densities (≦10⁸/cm²) are highly desirable for devices. The growth of structures with epitaxial layer thicknesses in excess of 1 micrometer without cracking can be problematic as well with conventional growth techniques.

Wang et al. (T. Wang, J. Bai, P. J. Parbrook, and A. G. Cullis, “Air-bridged lateral growth of an Al_(0.98)Ga_(0.02)N layer by introduction of porosity in an AlN buffer,” Appl. Phys. Lett. vol. 87 (2005) pp. 151906-1-151906-3) reports significant dislocation reduction by introduction of a porous AlN buffer via metalorganic chemical vapor deposition. An approximately 500-nm AlN layer with a high density of pores was directly grown on a sapphire substrate at 1150° C. An important feature is pointed out that, although the column-like islands do not coalesce, the top of each island is very flat. Decreasing temperature and increasing V/III ratio can increase the area of the pores, resulting in a rough surface for the high-Al-composition AlGaN layer subsequently grown on the porous layer.

Kida et al. (Y. Kida, T. Shibata, H. Miyake, and K. Hiramatsu, “Metalorganic Vapor Phase Epitaxy Growth and Study of Stress in AlGaN Using Epitaxial AlN as Underlying Layer,’Jpn. J. Appl. Phys. Vol. 42, (3002) pp. L572-L574) report the growth of AlGaN on an AlN underlying layer. An epitaxial AlN film of 1 micrometer thickness grown on sapphire (0001) by low-pressure metalorganic vapor phase epitaxy (LO-MOVPE) was used as a substrate; it had an atomically flat surface.

Hiramatsu et al. (K. Hiramatsu, H. Miyake, H. Yoshida, T. Urushido, and Y. Terada, “Group III nitride semiconductor film and its production method,” U.S. Patent Application No. 20040157358) reports a group III nitride semiconductor film involving few lattice defects and having a large thickness, and a process for making the film. Dry-etching is conducted while a quartz substrate and a group III nitride semiconductor are placed on the top of a lower electrode. Nano-pillars are formed on the top of the group III nitride semiconductor. Another group III nitride semiconductor film is deposited on the nano-pillars.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form part of the specification, illustrate some embodiments of the present invention and, together with the description, serve to explain the principles of the invention.

FIG. 1 illustrates several embodiments of the invention comprising a substrate and a denticulated group III nitride layer.

FIG. 2 illustrates an embodiment comprising a 1.2-micrometer-thick AlN denticulated layer. FIGS. 2 a and 2 b are atomic force microscope (AFM images);

FIG. 2 c is a scanning electron micrograph (SEM) image.

FIG. 3 illustrates the dependence of several embodiments comprising AlN denticulated surfaces on the denticulated layer thickness. a) layer thickness=0.11 micrometer: 20 nm/vertical division, rms 1.6 nm; b) layer thickness=0.25. micrometers: 40 nm/vertical division, rms 3.3 nm; c) layer thickness=0.47. micrometers: 100 nm/vertical division, rms 13.1 nm; d) layer thickness=0.80. micrometers: 300 nm/vertical division, rms 27.9 nm; e) layer thickness=1.2. micrometers: 250 nm/vertical division, rms 27.7 nm; 3×3 micrometer sample size for all. The denticulated layers were grown at 970° C. using a pressure of 75 Torr and a V/III ratio of 121, a H₂ flow of 9.5 slpm, and a molar flux of 11.9×10⁻⁶ moles/min of trimethyl aluminum.

FIG. 4 illustrates a graph of rms roughness versus AlN denticulated layer thickness for 1×1 and 3×3 micrometer regions of the samples shown in FIG. 3.

FIG. 5 illustrates the effect of temperature on the surface texture of an AlN denticulated surface. Growths were performed at 75 Torr, 11.9×10⁻⁶ moles/min of trimethylaluminum (TMA), V/III ratio of 121, H₂=9.5 slpm. Temperatures were a) 940° C.; b) 970° C.; c) 1000° C.

FIG. 6 illustrates the effect of V/III ratio on the surface texture of an AlN denticulated surface. Growths were performed at 75 Torr, trimethyl aluminum (TMA)=33 sccm, H₂=9.5 slpm. a) NH₃=27 sccm, V/III=50; b) NH₃=65 sccm, V/III=121; c) NH₃=180 sccm, V/III=336.

FIG. 7 illustrates the effect of hydrogen carrier gas flow on the surface texture of an AlN denticulated surface. Growths were performed at 75 Torr, 11.9×10⁻⁶ moles/min of trimethylaluminum (TMA), V/III ratio of 121 H₂ flows were a) 2 slpm; b) 9.5 slpm; c) 17 slpm.

FIG. 8 illustrates the reduction of the density of edge-component threading dislocation in two samples of high-temperature AlN grown on denticulated AlN layers as a function of increasing denticulated layer thickness (AlN transition layer thickness).

FIG. 9 is a transmission electron micrograph (TEM) image of a structure comprising a denticulated AlN layer that has had a high-temperature AlN layer grown upon it. The AlN denticulated layer was 0.43 micrometer thick. The high-temperature AlN layer was grown at 1070° C. to a thickness of 1.5 micrometers. Threading dislocations are seen to interact and be removed in the 0.6 micrometer region grown on top of the denticulated layer at 1070° C. where the AlN is coalescing.

FIG. 10 shows TEM images of an Al_(0.63)Ga_(0.67)N layer that has been grown on top of a high-temperature AlN layer that was grown on top of a denticulated AlN layer. a) View showing dislocations with an edge component. b) View showing dislocations with a screw component. Mixed dislocations are part screw and part edge in character.

DETAILED DESCRIPTION OF THE INVENTION

This invention comprises a denticulated Group III nitride structure that is useful for growing Al_(x)Ga_(1-x)N to greater thicknesses without cracking and with a greatly reduced threading dislocation (TD) density as compared with conventionally grown materials. Methods for growing embodiments of the invention comprise growing a denticulated Group III nitride layer. In some embodiments, a moderate temperature is employed during the growth of the denticulated layer. In some embodiments, this denticulated layer is situated as a transitional layer between a lower temperature nucleation layer and Group III nitride layer grown under higher temperature conditions typically employed for that Group III nitride material.

Some embodiments of denticulated Group III nitride layers are illustrated in FIG. 1. The structure illustrated in FIG. 1 a comprises a substrate 1 and a denticulated Group III nitride layer 2. The structure illustrated in FIG. 1 b comprises a substrate 1, a nucleation layer 3, and a denticulated layer 2. FIGS. 1 c (without nucleation layer 3) and 1 d (further comprising a nucleation layer 3) illustrate structures comprising a substrate 1, a denticulated layer 2, and a layer of Group III nitride 4 grown at higher temperature on the denticulated layer. The dotted lines are intended to represent the surface of the denticulated layer 2 prior to the growth of the higher temperature layer 4 upon the denticulated layer. Growth of additional group III nitride on the denticulated surface at the higher temperature layer leads to lateral growth and coalescence that obscures the previous location of the denticulated surface structure.

FIG. 2 illustrates a denticulated Group III nitride layer surface that is an embodiment of this invention. The embodiment illustrated in FIG. 2 comprises a 1.2-micrometer-thick AlN denticulated layer. FIGS. 2 a and 2 b are atomic force microscopy (AFM) images with FIG. 2 a showing a 1×1 micrometer surface region with an rms roughness of 23.8 nm for the imaged area. FIG. 2 b shows a 3×3 micrometer region of the same layer and has an rms roughness of 27.7 nm. FIG. 2 c is a scanning electron micrograph (SEM) of the surface of the same sample.

FIG. 3 presents AFM images of the surface of several embodiments of the denticulated layer as a function of layer thickness. FIG. 4 graphs the rms roughness for the layers illustrated in FIG. 3 as a function of layer thickness. For thin layers (<0.3 micrometer), the formation of substantially similar denticles has not been achieved, as shown in FIGS. 3 a and 3 b. For the illustrated layer thickness of 0.43 and thicker, a substantially similar denticulated surface has been achieved. Embodiments of this invention that generate surfaces covered with denticles substantially similar to those in FIGS. 3 c, 3 d, and 3 e, that is, generally tooth-like, can be used as growth substrates for subsequently high-temperature Group III nitride growth where the thickness for onset of cracking is substantially increased. Denticulated surfaces substantially similar to those shown in FIGS. 3 c, 3 d, and 3 e also produce a reduction in the edge-component threading dislocation density in the high-temperature Group III nitride layers grown on top of them. Denticulate layers are grown at moderate temperatures, as described below, rather than at the high temperatures typically employed for Group III nitride growth.

The growth conditions for the denticulated group III nitride layer in some embodiments are as follows. A growth pressure of 75 Torr is used for some embodiments; pressures up to 300 Torr have been used in some embodiments. Higher pressures, up to 1000 Torr, and lower pressures, down to 40 Torr, may also be used. In many embodiments, the transitional layer is grown at approximately 970° C.; other embodiments can employ temperatures between 900° C. and 1020° C. This temperature range is defined as moderate temperature. Temperatures in excess of about 1000° C. are typically used in what is termed “high temperature” growth. FIG. 5 illustrates the dependence of surface structure on temperature for growth conditions of 75 Torr, 11.9×10⁻⁶ moles/min of trimethylaluminum (TMA), V/III ratio of 121, H₂=9.5 slpm in a commercial short-jar nitride metalorganic chemical vapor deposition (MOCVD) reactor. The V/III ratio is calculated using the molar flux of NH₃ (@ STP, 22.4 slpm=1 mole of NH₃, 6.023×10²³ N atoms per mole of NH₃) and 2 times the molar flux of TMA to account for the diatomic nature of the TMA molecule as it is transported in the vapor phase. The combination of gas flows and pressures that will properly produce the denticulated structure at a particular moderate temperature will be partially dependent on the operating characteristics of a particular reactor and can be determined without undue experimentation by those skilled in the MOCVD art. Other growth techniques, such as hydride vapor phase epitaxy, can also be used in embodiments of this invention. The growth conditions for embodiments reported herein are suitable for the reactor employed; some modification may be required for other reactors. When a surface looks like FIG. 5 a, the denticulate structure has not sufficiently formed. FIG. 5 b illustrates a well-formed denticulated surface that works well for reducing cracking and threading-dislocation density in subsequent high-temperature growths. FIG. 5 c shows a surface that is characterized by vertically oriented platelets, sometimes with extended interconnection between adjacent plates producing plateau-like features, rather than by substantially conical denticles. Embodiments of this invention have denticulated surfaces, as in FIG. 5 b, rather than the more plateau-like features shown in the higher-temperature surface of FIG. 5 c. The specific temperature where the denticulated surface is optimized will depend somewhat on the reactor conditions, but should be in most embodiments between 940 and 1000° C.

In some embodiments, an ammonia flow of 65 sccm or a V/III ratio of 121 are used for a growth rate of approximately 0.3 micrometers/hour. FIG. 6 illustrates the effect of ammonia flow when growing the denticulated layer at 970° C. When growing at an approximately 0.3 micrometer/hour rate, a V/III ratio of 50 is sufficiently low that less surface roughness is obtained. Under these conditions, a surface that is characterized by vertically oriented platelets, sometimes with extended interconnection between adjacent plates producing plateau-like features, rather than by substantially conical denticles, is obtained. Increased ammonia flow produced a denticulated surface (FIG. 6 b.) In some embodiments, a V/III ratio as high as 336 has provided growths with good surface roughness. FIG. 6 c shows a substantially denticulated surface. It is likely that higher V/III ratios will also work well as long as a substantially denticulated surface is formed. For AlN, trimethyl aluminum (TMA) has been employed as the group III precursor. Other Al precursors, such as triethylaluminum, dimethylethylamine alane, trimethylamine alane, dimethylaluminum hydride and tritertiarybutylaluminum may also be used. For group III nitrides comprising Ga as a group III element, trimethylgallium (TMG) and triethylgallium, and triisopropylgallium may be used. Successful growth of a denticulated surface has been achieved for V/III ratios from approximately 120 to 270. However, some reduction in dislocation density and improved resistance to cracking should be achieved from layers grown with V/III ratios down to 25 and higher than 330.

In some embodiments, growth rates on the order of 1 micrometer/hour have produced good surface roughness; other growth rates may also be used provided they correspond to a surface with an appropriate level of roughness. Growth rates and conditions that produce denticulated surfaces are good for the purpose of dislocation and cracking reduction.

In some embodiments, hydrogen gas flow rates of 10-13 slpm are used; flows down to approximately 2 slpm and up to approximately 17 slpm have provided good surface roughness. FIG. 7 illustrates the AFM images of surfaces growth at 970° C. with different hydrogen flow rates. Lower or higher hydrogen flow rates can be used in some embodiments if they produce substantially denticulated surfaces.

The formation of a denticulated surface leads to marked reduction in threading dislocations and an increase in the thickness that can be grown without cracking. Reaction conditions that produce short ridges, as shown in FIGS. 5 c and 6 a may provide some improvement, but these surfaces do not produce as great a benefit as is obtained with a denticulated surface.

In some embodiments, a transitional-layer thickness between 100 nm and 500 nm are employed. In other embodiments, transitional-layer thicknesses between 40 nm and 1.2 micrometers in thickness can be used. In other embodiments, transitional-layer thicknesses between 0.4 micrometers and 1.2 micrometers can been used to produce decreases in threading dislocation density and increases in the thickness of group III nitride that can be grown on top of the denticulated layer before cracking occurs. Thicker layers may also be useful provided they have a denticulated surface texture. Considerations in selecting the thickness of the denticulated layer include the dislocation density that is required for a particular application and the amount of time required to grow the layer. When growing at moderate temperatures, a rough surface is generated during layer growth and, below a certain transitional-layer thickness, the surface becomes rougher as the layer grows. Past a certain thickness depending on growth conditions, the roughness may remain approximately constant as the transitional layer thickness increases over a range depending on growth conditions. High-temperature group III nitride layers grown on thicker denticulated layers have lower in-plane strain; this permits growth of thicker high-temperature layers before the strain causes cracking of the layer. As the transitional layer thickness increases, a thicker layer of high-temperature-grown III-N can be grown on top of the transitional layer before cracking is observed. For example, AlN films grown on sapphire at high temperature (for example, 1100° C.) without an AlN denticulated transitional layer typically form cracks at thicknesses between 0.7 and 1 micrometer. Based on residual strain of the high temperature AlN layer measured by x-ray diffraction, a transitional layer of 0.3 micrometer thickness permits growth of a 1.4-micrometer high-temperature layer before cracking. This transitional-layer structure has a tensile strain of 1.6×100.3 at the growth temperature. Thicker transitional layers permit growth of thicker crack-free layers upon the transitional layer. For example, 4 micrometers of AlN have been grown on a 0.75-micrometer-thick denticulated transitional layer. The strain is much lower (4×10⁻⁴) and a representative calculation predicts that a layer of 37 micrometers would be possible before cracking with such a low strain value.

In addition to an increased thickness before cracking, the high-temperature AlN layer has a much lower threading dislocation density. A representative aluminum nitride film grown without a transitional layer had a density of dislocations with an edge component of 1.2×10¹⁰/cm² and was restricted to a thickness of 0.76 micrometers to avoid cracking. A 3-micron AlN layer grown with the transitional layer having a thickness of approximately 0.45 micrometers has been determined by X-ray diffraction to contain approximately 1-3×10⁸/cm² screw-component threading dislocations and 2-3×10⁹ cm² edge-component threading dislocations. An AlN layer grown on an 0.88 micrometer AlN denticulated transitional layer displayed a density of screw-component threading dislocations of approximately 1.1×10⁸/cm²) and a density of edge-component threading dislocations of 9.5×10⁸ cm² based on X-ray diffraction measurements. Surfaces of the high-temperature AlN layer grown on a denticulated transitional layer are featureless under Nomarski microscopy and exhibit AFM RMS values less than 0.7 nm over 10×10 square micrometer areas. FIG. 8 illustrates the dependence of the edge-component threading dislocation density on AlN denticulated layer thickness (transition layer thickness) for high-temperature AlN layers of 1.5 and 1.8 micrometer grown at 1100° C. The method used to determine dislocation density using X-ray diffraction is described in S. L. Lee, Appl. Phys. Lett. vol. 86 (2005) p 241904.

In several embodiments, an AlN nucleation layer of approximately 10 nm thickness was grown on a sapphire substrate. In some embodiments, the nucleation layer is grown at a low temperature, typically between 400 and 700° C. A denticulated AlN transitional layer was grown to a thickness of approximately 0.4 micrometers using a pressure of 75 Torr, a V/III ratio of 121 with the V source being ammonia and the III source being trimethyl aluminum, a group-III flux between 16 and 95 micromoles/min, and a growth temperature of 970° C. The hydrogen carrier gas flow rate was between 10 and 13 slpm. A high-temperature AlN layer was grown on the denticulated transitional layer using a pressure of 75 Torr, a V/III ratio between 6 and 60, a group III flow rate between 16 and 95 micromoles/min. The high-temperature AlN layer was grown to a thickness of about 1.5 micrometers without cracking. The ultimate dislocation density of the high temperature AlN film grown on the denticulated transitional layer is dependent on its growth conditions and thickness in addition to the growth conditions and thickness of the transitional layer. Growing a transitional layer with the conditions above to a thickness of 0.3 to 0.8 micrometers will typically result in high temperature AlN films (1.5 to 3 micrometer thick) with a density of dislocations with an edge component of 1-6×10⁹/cm² (measured by x-ray diffraction). AlN films with lower dislocation density can be achieved by optimization of the growth conditions for the interlayer and especially by growing a thicker transitional layer and high temperature AlN layer.

FIG. 9 presents a transmission electron micrograph (TEM) image that shows the reduction of edge-component threading dislocation density in high-temperature AlN grown on a denticulated layer of 0.43 micrometer thickness. In the 0.6 microns of AlN grown at 1070° C. on top of the denticulated layer, the dislocations are seen to interact and be reduced, resulting in a lower dislocation density in the 0.9 microns of high-temperature AlN above the interaction zone.

FIG. 10 illustrates the use of the denticulated layer as a basis for forming a low-dislocation substrate of AlN for subsequent growth of AlGaN. Dislocations react and are removed in the first 0.6 micrometers of AlN grown at 1070° C. on top of a denticulated layer grown at 1000° C. A layer of Al_(0.63)Ga_(0.37)N was grown on the high-temperature AlN layer to produce lower-dislocation density AlGaN suitable for use in devices.

In some embodiments of this invention, an additional denticulated transitional layer may be grown on the high-temperature AlGaN layer, followed by additional growth of high-temperature AlGaN, to produce further improvements in dislocation density and crack-free thickness. In some embodiments, denticulated layers can be grown on patterned substrates or on patterned substrates comprising layers of GaN, AlGaN or AlN where the substrates have been patterned to form various structures on their surface. Some examples of such structures include but are not restricted to periodic trenches, 1-dimensional or 2-dimensional arrays of grooves and mesas such as, for example a periodic structure that is square-wave in cross section, grooves between pyramidal structures, hexagonal structures such as, for example, 2-dimensional arrays of hexagonal islands with flat mesa tops, 6-sided trapezoidal islands, 6-sided pyramidal structures, and other polygonal structures. The structures can be formed by photolithography and etching or by previous growth conditions. In such embodiments, the patterned surfaces form what is termed a textured substrate. The resulting structure in AlN or AlGaN can also be used to reduce dislocation density and cracking in overgrown AlGaN films.

Embodiments of this transitional layer can use a variety of different thin nucleation layers, as are known to those skilled in the art of III-nitride growth. The AlN or Al_(x)Ga_(1-x)N (0≦x≦1) layer that is grown on top of this transitional layer can be grow under many different conditions as are known to those skilled in the art. Embodiments of this invention have been demonstrated using sapphire and SiC substrates; other substrate materials suitable for growth of III-N materials can also be used in embodiments of this invention. Examples include but are not restricted to sapphire, silicon carbide, silicon, lithium gallate, lithium aluminate, zinc oxide, gallium nitride, aluminum nitride, spinel (MgAl₂O₄), hafnium (Hf), and zirconium diboride.

Transitional layers made using embodiments of this invention differ in their behavior from previously reported rough layers of GaN. There is significant mass transport and etching in the growth of GaN. In forming previously reported rough GaN, the previously deposited GaN layer is etched and Ga atoms are redistributed to form large islands that are responsible for a reduction in dislocation density. The roughness scale of a GaN layer is often similar to its total thickness and the GaN is not necessarily a continuous film even with the deposition of additional material. In contrast, the denticulated layer of AlN or high-Al-content Al_(x)Ga_(1-x)N grown in embodiments of this invention grows thicker at an appreciably faster rate than the rate at which it roughens. The denticulated layer films of embodiments of this invention are growing films with surfaces that initially get rougher as the films grow thicker and then exhibit a substantially constant level of roughness as the film grows still thicker.

Embodiments of denticulated layers for reduced threading dislocation densities and cracking can be applied to the growth of electronic and photonic devices to obtain improved performance. In one embodiment, Al_(x)Ga_(1-x)N (0≦x≦1) layers grown on denticulated AlN layers may form the n-type Group III nitride cladding layer of a Group III nitride p-i-n light emitting diode or laser diode structure. Subsequent growth of a Group III nitride quantum well active region and p-type Group III nitride cladding layers comprising Al_(x)Ga_(1-x)N (0≦x≦1) or In_(x)Ga_(1-x)N (0≦x≦1) materials completes the light emitting structure. The reduced dislocation densities that are enabled by the formation of the denticulated layer before growth at higher temperature can improve device performance through increased optical output powers and device lifetimes. Furthermore, the ability to grow thicker cladding layers without cracking enables better optical mode confinement in Group III nitride laser diode structures and reduced lateral resistance in both light emitting diodes and laser diodes grown on insulating substrates. Other device structures that may benefit from the reduced dislocation densities and cracking afforded by this denticulated growth approach include Group III nitride photodetectors and transistors.

The preceding are illustrative embodiments. Specific combinations of reaction parameters to embody this invention in a different reactor will depend on the specific characteristics of the reactor employed. Persons of ordinary skill in the art with typical understanding of the operation of their own reactors are capable of determining suitable corresponding conditions for their reactors without undue experimentation.

-   -   The foregoing description of the invention has been presented         for purposes of illustration and description and is not intended         to be exhaustive or to limit the invention to the precise form         disclosed, and obviously many modifications and variations are         possible in light of the above teaching. The embodiments were         chosen and described in order to best explain the principles of         the invention and its practical application to thereby enable         others skilled in the art to best utilize the invention in         various embodiments and with various modifications as are suited         to the particular use contemplated. It is intended that the         scope of the invention be defined by the claims appended hereto. 

1. A method for growing a Group III nitride structure with reduced threading dislocation density and cracking, the method comprising: providing a substrate in a reactor; growing a denticulated Group III nitride layer on the substrate using denticulated layer growth conditions at a temperature between approximately 900° C. and approximately 1020° C.; and growing a reduced-dislocation-density Group III nitride layer atop the denticulated Group III nitride layer using reduced-dislocation-density growth conditions.
 2. The method of claim 1, further comprising growing a nucleation layer at low temperature, the nucleation layer being located between the substrate and the denticulated layer.
 3. The method of claim 2, wherein the nucleation layer comprises AlN grown at a temperature below approximately 700° C.
 4. The method of claim 1, wherein the steps of growing the denticulated Group III nitride layer and of growing the reduced-dislocation-density Group III nitride layer are performed sequentially without removing the Group III nitride structure from the reactor.
 5. The method of claim 1, wherein the step of growing the reduced-dislocation-density Group III nitride layer is performed at a temperature greater than approximately 1000° C.
 6. The method of claim 1, wherein the step of growing the denticulated Group III nitride layer is performed using a V/III ratio between approximately 25 and approximately
 330. 7. The method of claim 6 wherein the step of growing the denticulated Group III nitride layer is performed using a V/III ratio between approximately 120 and
 270. 8. The method of claim 1, wherein a thickness of the denticulated Group III nitride layer is greater than or equal to approximately 0.04 micrometers.
 9. The method of claim 8, wherein the thickness of the denticulated Group III nitride layer is less than or equal to approximately 1.2 micrometers.
 10. The method of claim 1, wherein the denticulated Group III nitride layer comprises Al_(x)Ga_(1-x)N wherein 0.25≦x≦1.
 11. The method of claim 1, wherein the substrate comprises a material selected from the group consisting of sapphire, silicon carbide, silicon, lithium gallate, lithium aluminate, lithium nitrate, zinc oxide, gallium nitride, aluminum nitride, spinel (MgAl2O4), hafnium (Hf), and zirconium diboride.
 12. The method of claim 1, wherein the substrate comprises a textured substrate.
 13. The method of claim 1, wherein the step of growing the reduced-dislocation-density Group III nitride layer comprises growing Group III nitride laterally from denticles of the denticulated Group III nitride layer to form a coalesced Group III nitride layer.
 14. The method of claim 13, further comprising growing Group III nitride atop the coalesced Group III nitride layer.
 15. The method of claim 1, wherein the steps of growing the denticulated Group III nitride layer and the reduced-dislocation-density Group III nitride layer employ a process selected from metal organic chemical vapor deposition and hydride vapor phase epitaxy.
 16. The method of claim 1, wherein the steps of growing the denticulated layer growth conditions and the reduced-dislocation-density growth conditions comprise using a Group III metal precursor and ammonia.
 17. The method of claim 16, wherein the Group III metal precursor is selected from the group consisting of triethylaluminum, dimethylethylamine alane, trimethylamine alane, trimethylaluminum, dimethylaluminum hydride, tritertiarybutylaluminum, trimethylgallium, triethylgallium, and triisopropylgallium.
 18. The method of claim 16, wherein the steps of growing the denticulated layer growth conditions and the reduced-dislocation-density growth conditions comprise using a pressure between approximately 40 Torr and approximately 1000 Torr.
 19. The method of claim 1, further comprising: growing an n-type Group III nitride cladding layer; growing a Group III nitride quantum well proximate to the n-type Group III nitride cladding layer; and growing a p-type Group III nitride cladding layer proximate to the Group III nitride quantum well. 